1. Field of the Invention
The present invention relates to a light emitting diode and a method for fabricating the same, in more particularly, to a high brightness light emitting diode and a method for fabricating the same, by which an emission of a near-infrared light in a distributed Bragg reflector can be reduced to a negligible level, and a malfunction of an optical sensor, etc. using an infrared light can be prevented.
2. Description of the Related Art
Recently, a demand of high brightness light emitting diodes for red light to green light, which are fabricated by using an AlGaInP-based epitaxial wafer, is largely developed. Main demands are backlight for a liquid crystal display of a cellular phone, indication lamp, signal lamp for traffic, brake lamp of automobile, etc. The AlGaInP is a direct transition type semiconductor having the largest bandgap among group III-V compound semiconductor other than nitrides. Therefore, comparing with a conventional light emitting diode using an indirect transition type semiconductor such as GaP or AlGaAs, it is possible to realize a high brightness emission in a visible wavelength range corresponding to green light from red light by using the AlGaInP. In addition, the high brightness light emitting diode that is generally manufactured and sold has extremely high internal quantum efficiency. Accordingly, so as to realize a higher brightness than the conventional high brightness light emitting diode, it is effective to improve an external quantum efficiency rather than the internal quantum efficiency. As means for improving the external quantum efficiency, a light emitting diode in which a distributed Bragg reflector (DBR) is inserted is proposed.
FIG. 1 shows a structure of a conventional AlGaInP-based light emitting diode having an emission wavelength of 630 nm, which is disclosed by Japanese Patent Laid-Open No. 2003-218386 (JP-A-2003-218386).
In the conventional AlGaInP-based light emitting diode, as shown in FIG. 1, on a n-type GaAs substrate 22, a distributed Bragg reflector (DBR) 23 comprising a multi-layered film which is made by layering a high refractive index film and a low refractive index film alternately, a n-type AlGaInP lower cladding layer 24, an undoped AlGaInP active layer 25, a p-type AlGaInP upper cladding layer 26, and a p-type GaP current dispersion layer 27 are sequentially grown by metalorganic vapor phase epitaxy (MOVPE method), and a backside electrode (n-side common electrode) 21 is provided on a whole backside surface of the n-type GaAs substrate 22, and a surface electrode (p-side Ohmic contact electrode) 28 is provided on a part of a surface of the p-type GaP current dispersion layer 27. The n-type AlGaInP lower cladding layer 24, the undoped AlGaInP active layer 25, and the p-type AlGaInP upper cladding layer 26 constitute an AlGaInP quaternary double hetero structure part (light emitting part).
The distributed Bragg reflector 23 comprises a multi-layered film which is made of alternatively layering a high refractive index having a film thickness of λ/4 n and a low refractive index film having a film thickness of λ/4 n, wherein an emission wavelength of the LED is λ and a refractive index is n. The distributed Bragg reflector 23 has a function of reflecting a light advancing to a lower direction (a direction of the GaAs substrate) which is a part of a light generated in the active layer to an upper direction (a direction for taking out the light) thereby improving an efficiency for taking out the light. By this effect, an improvement in the brightness for 50% or more (about 100% in the case) can be realized, in comparison with the LED in which the distributed Bragg reflector is not interposed.
In the LED comprising an AlGaInP mixed crystal as a light emitting layer, a combination of GaAs layer and AlxGa1−xAs layer (0≦X≦1), a combination of GaAs layer and (AlxGa1−x)1−yInyP layer (0≦X≦1, 0≦y≦1), etc. are generally used as materials constituting the distributed Bragg reflector 23. In addition, as a material constituting the distributed Bragg reflector 23, a combination of AlAs layer and AlGaAs layer having a higher refractive index and higher reflective index is known.
On the other hand, in JP-A-2003-218386, a distributed Bragg reflector comprising a combination of the GaAs layer and AlInP layer is used. The reasons therefor are explained as follows. In the distributed Bragg reflector comprising the combination of the AlAs layer and AlGaAs layer, a manufacturing condition of the AlAs layer is very difficult, and it is easily contaminated with oxygen (O), so that a crystallinity of the light emitting layer grown thereon is deteriorated remarkably, thereby causing a deterioration of the brightness. In addition, if a method for increasing a supplying amount ratio of group V source and group III source (so-called V/III ratio) is adopted so as to prevent the oxygen mixture, a heavy load is charged on the manufacturing apparatus, so that a discharge pipe is easily blocked with As dusts.
Accordingly, in the distributed Bragg reflector comprising the GaAs layer and AlInP layer shown by JP-A-2003-218386, it is proposed that a thickness of the GaAs layer is made as thin as possible, such that a light absorbed in the GaAs layer is reduced to provide a high reflective index, while the AlInP layer is made thick, such that a light having a desired wavelength to be reflected can be reflected.
A film thickness t1 of the GaAs layer and a film thickness t2 of the AlInP layer required for manufacturing such a distributed Bragg reflector with a high reflective index can be determined by following formulas:t1={λ0/(4×n1)}×α,t2={λ0/(4×n2)}×(2−α), and0.5<α<0.9
wherein t1 is a film thickness [nm] of the GaAs layer,
t2 is a film thickness [nm] of the AlInP layer, λ0 is a wavelength [nm] of a light to be reflected, n1 is a refractive index of the GaAs layer to the wavelength of the light to be reflected, and n2 is a refractive index of the AlInP layer to the wavelength of the light to be reflected.
The distributed Bragg reflector is manufactured by layering alternately the GaAs layer and AlInP layer each having a film thickness thus calculated for several dozens of times.
As a material composing the distributed Bragg reflector 23 other than the aforementioned examples, Japanese Patent Laid-Open No. 2000-174332 (JP-A-2000-174332) discloses that AlGaAs is used for both of the low refractive index film and the high refractive index film.
However, there is a major problem in the LED using the distributed Bragg reflector comprising the combination of the GaAs layer and AlInP layer disclosed by the JP-A-2003-218386. In such a LED, a light having a wavelength of 630 nm is emitted from the active layer as a main light (main peak in a light emission of the LED) shown in FIG. 2. In addition to this main light, a near-infrared light having an emission wavelength of 860 nm is unintentionally emitted from the GaAs layer due to a light advanced from the active layer to the distributed Bragg reflector as shown in FIG. 2 (the emission of the LED) The near-infrared light thus occurred has a considerable intensity, and this near-infrared light is emitted to the outside together with the main light emitted from the active layer.
The intensity of the near-infrared light having the emission wavelength of 860 nm is about 1/10 of the main light having the emission wavelength of 630 nm. This near-infrared light may cause malfunction of a sensor using the infrared light, which is widely spread.
Further, in the distributed Bragg reflector comprising the combination of the GaAs layer and AlInP layer disclosed by JP-A-2003-218386, there is a further disadvantage in that a part of the light is absorbed by the GaAs layer without being reflected, so that the LED becomes darkened (the brightness of the LED is deteriorated).
Accordingly, it is an object of the invention to provide a high brightness light emitting diode and a method for fabricating the same, in which an emission of a near-infrared light in a distributed Bragg reflector is reduced to a negligible level, and a light absorptance in the distributed Bragg reflector is lowered.
So as to achieve the object of the invention, the present invention is characterized by following features.
According to a first feature of the invention, a light emitting diode, comprises:
a first conductivity type substrate;
a first conductivity type distributed Bragg reflector comprising a multi-layered film which is made by layering a high refractive index film composed of an AlGaAs layer and a low refractive index film composed of an AlInP layer as alternately; and
a light emitting part comprising an active layer sandwiched between a first conductivity type lower cladding layer and a second conductivity type upper cladding layer.
According to a second feature of the invention, in the light emitting diode, the first conductivity type distributed Bragg reflector is interposed between the first conductivity lower cladding layer and the first conductivity type substrate.
According to a third feature of the invention, the light emitting diode, further comprises:
a second conductivity type current dispersion layer grown on the light emitting part.
According to a fourth feature of the invention, in the light emitting diode, a film thickness of each of the AlGaAs layer and AlInP layer constituting the first conductivity type distributed Bragg reflector is determined by following formulas (1) to (3):t1={λ0/(4×n1)}×α  (1),t2={λ0/(4×n2)}×(2−α)  (2), and0.5<α<0.9  (3)
wherein t1 is a film thickness [nm] of AlGaAs layer, t2 is a film thickness [nm] of AlInP layer, λ0 is a wavelength [nm] of a light to be reflected, n1 is a refractive index of the AlGaAs layer to the wavelength of the light to be reflected, and n2 is a refractive index of the AlInP layer to the wavelength of the light to be reflected.
According to a fifth feature of the invention, in the light emitting diode, the high refractive index film composed of AlGaAs layer is expressed as AlxGa1−xAs layer wherein Al mixed crystal ratio x is 0<x<0.6.
According to a sixth feature of the invention, in the light emitting diode, the first conductivity type substrate is GaAs, and the light emitting part is composed of AlGaInP or GaInP.
According to a seventh feature of the invention, a method for fabricating a light emitting diode, comprises steps of:
providing a first conductivity type substrate;
providing a first conductivity type distributed Bragg reflector by growing an AlGaAs layer and an AlInP layer alternately on the first conductivity type substrate by a MOVPE method; and
sequentially growing a first conductivity type AlGaInP cladding layer, an undoped AlGaInP active layer, a second conductivity type AlGaInP cladding layer, and a second conductivity type current dispersion layer by the MOVPE method;
wherein:
a film thickness of each of the AlGaAs layer and AlInP layer constituting the first conductivity type distributed Bragg reflector is determined by following formulas (1) to (3):t1={λ0/(4×n1)}×α  (1),t2={λ0/(4×n2)}×(2−α)  (2), and0.5<α<0.9  (3)
wherein t1 is a film thickness [nm] of the AlGaAs layer, t2 is a film thickness [nm] of the AlInP layer, λ0 is a wavelength [nm] of a light to be reflected, n1 is a refractive index of the AlGaAs layer to the wavelength of the light to be reflected, and n2 is a refractive index of the AlInP layer to the wavelength of the light to be reflected.